RPL MRHOF with ETX: bypass weighted moving average when setting a parent's link metric for the first time

Currently the SPI driver in core/ sets `SPI_FLUSH()` to be a single read
of the spi RX buffer. This is fine for many microcontrollers, but newer
platforms like the CC2538 have a multi-byte FIFO for the RX buffer. In
that case, a single read is not guaranteed to flush the RX. This commit
allows `SPI_FLUSH()` to be platform dependent if needed, but doesn't
change for platforms where it works.

Ensure that the objective function's reset function is called at least once. This will enable the objective function to initialize itself during run-time.

Use the same scale for ETX calculation as for rank calculations to avoid conversion errors between the two.

Bugfix: when a packet is sent by the RDC layer, we should count the number of transmissions made because a single callback can pertain to multiple transmissions at the RDC layer.

Don't include a sliding window in the ICMPv6 datagram
unless the window has at least one active datagram
associated with it

  - init()
  - process incoming multicast datagram
  - Pass ICMPv6 trickle messages to the engine

We store multicast routes in a separate table since we don't need
as much information as we need for normal routes

Pending IANA allocation, we currently use private experimentation

Ensure that the file end is updated in the original file extent if it has been overwritten through a micro log operation.